In recent years, business utilizing personal contents, such as the exchange of photos and video contents over Social Networking Service (SNS) websites, has been becoming more and more popular. In addition, as development of infrastructure supporting such exchange of personal contents progresses, opportunities are increasing where people belonging to a same group exchange with each other personal contents, such as photos taken at a travel destination and video contents taken at a school sports festival or a school play.
For instance, Patent Literature 1 discloses a technology as described in the following. A content management server checks, via a network, whether or not a content-using device is connected with the content management server. When the content-using device is connected with the content management server, the content management server transmits, via the network, a copy deletion command to the content-using device. The content deletion command includes a content ID corresponding to an invalidation-target content. The content-using device receives the copy deletion command from the content management server and executes deletion processing with respect to a copied content corresponding to the content ID included in the copy deletion command. In contrast, when the content-using device is not connected with the content management server, the content management server repeatedly tries to establish a connection with the content-using device until the deletion processing with respect to the copied content is completed. As such, the technology disclosed in Patent Literature 1 prevents the use of an unauthorized copied content.